It is often necessary or useful to know the mass of air flowing through a passageway. While there are many applications for an anemometer or air mass sensor, an application of particular interest is in an internal-combustion engine. For an automobile engine with electronic fuel injection and ignition systems, air mass flow into the engine is one of several important sensed conditions useful in generating an electrical signal which controls and optimizes performance of those systems.
One type of conventional air mass sensor utilizes a temperature-dependent resistive wire, such as platinum wire, having an electrical resistance proportional to its temperature. The resistive wire is placed in the air mass flow of a passageway, and an electrical circuit supplies electric current to the wire. The circuit automatically regulates the flow of current through the resistive wire to maintain its resistance and hence its temperature constant, and measures that current. The measured current (or a voltage proportional thereto) indicates the mass of air per unit of time flowing through the passageway, and is used by the circuit to generate an air mass flow-indicating signal. Similar types of air mass sensors utilize a temperature-dependent resistive metallic film.
While having proven advantages, it is difficult to mount a hot wire or film air mass sensor in the air induction system of an internal-combustion engine to produce accurate and reliable readings. In the past, a hot wire sensor has been mounted within a conduit leading the engine throttle body and forming a part of the engine air induction system. One such arrangement is shown in U.S. Pat. No. 4,523,461. While this arrangement works well, it is very desirable to locate the air mass sensor in a position where it directly measures the air mass flow as it actually passes through the throttle body. Moreover, it is desirable to sample the air mass flow in the central portion of the throttle body, and not just the air mass flow adjacent to one portion of the throttle body wall. Because of the harsh environment that exists in the throttle body during operation of the internal-combustion engine, it is not practical to merely position a hot wire or film sensor so that it extends across the throttle body throat.
In the past, throttle body bypass channels or passages in which a heated sensor is mounted have been used, such as shown in U.S. Pat. No. 4,264,961. With this arrangement, a bypass passage located to one side of the throttle body has an inlet port communicating with the interior air duct of the throttle body air duct of the throttle body toward its upper open end, and outlet port communicating with the throttle body air duct at about the venturi constriction. A heat generator is positioned in the bypass passage and is connected to a air flow meter which generates an output voltage signal.
The engine air flow over the venturi causes a depression in the static pressure at the bypass passage outlet port and the resulting pressure differential in the bypass passage causes a fixed fraction of the engine air mass flow to pass into the inlet port and over the sensor. The sensor measures this fraction of the engine air mass flow and from that the total engine air mass flow is deduced. This design, however, has several disadvantages. The inlet port comprises a single orifice located to one side of the throttle body and draws air from only a localized area within the throttle body air duct and adjacent to one portion of the air filter mounted on the throttle body. Should local variations in filter clogging affect the air flow in that localized area about the inlet port, the air mass flow measured is affected, and the measured flow is no longer in the same proportion to the total engine air mass flow. This produces inaccuracies and erroneous engine air mass flow readings.
A similar problem exists as a result of the outlet port being a single orifice located to one side of the throttle body. The air mass flow of the bypass passage is exhausted into only a localized area within the throttle body air duct of the throttle body and adjacent to one edge portion of the butterfly throttle plate valve located below the outlet port. As the butterfly valve opens and closes, the air flow past the butterfly is not even at all points about its perimeter, and the particular location selected for the outlet port affects the air mass flow through the bypass passage, and hence the accuracy of the air mass flow measured. As a general matter, the downside of the butterfly valve permits more flow than the upside of the valve. Since no one position for the outlet port of the bypass passageway is relative to the butterfly valve it is believed representative of the total engine air mass flow for all valve opening and closing positions, the measured flow is almost always inaccurate. This is in addition to any inaccuracy created by the fact that the engine air filter may be dirty in a spot which affects the accuracy.
Another disadvantage of using a bypass passage is its sensitivity to engine backflow pulsations that are very pronounced at low speed wide open throttle running. When air mass flows for even a short pulse in the reverse direction through the throttle body air duct, a pressure differential is created across the bypass passage inlet and outlet ports much as when the air mass flow is flowing in the normal flow direction toward the engine. This causes air to enter the inlet port and exit from the outlet port even though the actual flow in the throttle body air duct is reveresed. This air mass flow in the bypass passage which is not at all representative of the engine air mass flow, creates an air mass flow reading as it passes the heated sensor located in the passage. The air mass flow measured is not discernible from the engine air mass flow in the normal direction, thus gross errors in computing of the actual engine air mass flow can occur when a reverberation condition exists.
It will therefore be appreciated that there has been a significant need for a throttle body with an air mass sensor mounted therein in a manner which avoids the disadvantages described above, and which provides accurate readings of the air mass flow through the throttle body without being affected by dirty spots in the air cleaner, the position of the sensor relative to the butterfly, or engine backflow pulsations. The present invention fulfills these needs and further provides other related advantages.